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NAME | DESCRIPTION | NOTES | SEE ALSO |
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nscd.conf(5) File Formats Manual nscd.conf(5)
nscd.conf - name service cache daemon configuration file
The file /etc/nscd.conf is read from nscd(8) at startup. Each
line specifies either an attribute and a value, or an attribute,
service, and a value. Fields are separated either by SPACE or
TAB characters. A '#' (number sign) indicates the beginning of a
comment; following characters, up to the end of the line, are not
interpreted by nscd.
Valid services are passwd, group, hosts, services, or netgroup.
logfile debug-file-name
Specifies name of the file to which debug info should be
written.
debug-level value
Sets the desired debug level. 0 hides debug info. 1
shows general debug info. 2 additionally shows data in
cache dumps. 3 (and above) shows all debug info. The
default is 0.
threads number
This is the initial number of threads that are started to
wait for requests. At least five threads will always be
created. The number of threads may increase dynamically
up to max-threads in response to demand from clients, but
never decreases.
max-threads number
Specifies the maximum number of threads. The default is
32.
server-user user
If this option is set, nscd will run as this user and not
as root. If a separate cache for every user is used (-S
parameter), this option is ignored.
stat-user user
Specifies the user who is allowed to request statistics.
reload-count unlimited | number
Sets a limit on the number of times a cached entry gets
reloaded without being used before it gets removed. The
limit can take values ranging from 0 to 254; values 255 or
higher behave the same as unlimited. Limit values can be
specified in either decimal or hexadecimal with a "0x"
prefix. The special value unlimited is case-insensitive.
The default limit is 5. A limit of 0 turns off the
reloading feature. See NOTES below for further discussion
of reloading.
paranoia <yes|no>
Enabling paranoia mode causes nscd to restart itself
periodically. The default is no.
restart-interval time
Sets the restart interval to time seconds if periodic
restart is enabled by enabling paranoia mode. The default
is 3600.
enable-cache service <yes|no>
Enables or disables the specified service cache. The
default is no.
positive-time-to-live service value
Sets the TTL (time-to-live) for positive entries
(successful queries) in the specified cache for service.
Value is in seconds. Larger values increase cache hit
rates and reduce mean response times, but increase
problems with cache coherence. Note that for some name
services (including specifically DNS) the TTL returned
from the name service is used and this attribute is
ignored.
negative-time-to-live service value
Sets the TTL (time-to-live) for negative entries
(unsuccessful queries) in the specified cache for service.
Value is in seconds. Can result in significant
performance improvements if there are several files owned
by UIDs (user IDs) not in system databases (for example
untarring the Linux kernel sources as root); should be
kept small to reduce cache coherency problems.
suggested-size service value
This is the internal hash table size, value should remain
a prime number for optimum efficiency. The default is
211.
check-files service <yes|no>
Enables or disables checking the file belonging to the
specified service for changes. The files are /etc/passwd,
/etc/group, /etc/hosts, /etc/resolv.conf, /etc/services,
and /etc/netgroup. The default is yes.
persistent service <yes|no>
Keep the content of the cache for service over server
restarts; useful when paranoia mode is set. The default
is no.
shared service <yes|no>
The memory mapping of the nscd databases for service is
shared with the clients so that they can directly search
in them instead of having to ask the daemon over the
socket each time a lookup is performed. The default is
no. Note that a cache miss will still result in asking
the daemon over the socket.
max-db-size service bytes
The maximum allowable size, in bytes, of the database
files for the service. The default is 33554432.
auto-propagate service <yes|no>
When set to no for passwd or group service, then the
.byname requests are not added to passwd.byuid or
group.bygid cache. This can help with tables containing
multiple records for the same ID. The default is yes.
This option is valid only for services passwd and group.
The default values stated in this manual page originate from the
source code of nscd(8) and are used if not overridden in the
configuration file. The default values used in the configuration
file of your distribution might differ.
Reloading
nscd(8) has a feature called reloading, whose behavior can be
surprising.
Reloading is enabled when the reload-count attribute has a non-
zero value. The default value in the source code enables
reloading, although your distribution may differ.
When reloading is enabled, positive cached entries (the results
of successful queries) do not simply expire when their TTL is up.
Instead, at the expiry time, nscd will "reload", i.e., re-issue
to the name service the same query that created the cached entry,
to get a new value to cache. Depending on /etc/nsswitch.conf
this may mean that a DNS, LDAP, or NIS request is made. If the
new query is successful, reloading will repeat when the new value
would expire, until reload-count reloads have happened for the
entry, and only then will it actually be removed from the cache.
A request from a client which hits the entry will reset the
reload counter on the entry. Purging the cache using nscd -i
overrides the reload logic and removes the entry.
Reloading has the effect of extending cache entry TTLs without
compromising on cache coherency, at the cost of additional load
on the backing name service. Whether this is a good idea on your
system depends on details of your applications' behavior, your
name service, and the effective TTL values of your cache entries.
Note that for some name services (for example, DNS), the
effective TTL is the value returned from the name service and not
the value of the positive-time-to-live attribute.
Please consider the following advice carefully:
• If your application will make a second request for the same
name, after more than 1 TTL but before reload-count TTLs, and
is sensitive to the latency of a cache miss, then reloading
may be a good idea for you.
• If your name service is configured to return very short TTLs,
and your applications only make requests rarely under normal
circumstances, then reloading may result in additional load on
your backing name service without any benefit to applications,
which is probably a bad idea for you.
• If your name service capacity is limited, reloading may have
the surprising effect of increasing load on your name service
instead of reducing it, and may be a bad idea for you.
• Setting reload-count to unlimited is almost never a good idea,
as it will result in a cache that never expires entries and
puts never-ending additional load on the backing name service.
Some distributions have an init script for nscd(8) with a reload
command which uses nscd -i to purge the cache. That use of the
word "reload" is entirely different from the "reloading"
described here.
nscd(8)
Linux man-pages (unreleased) (date) nscd.conf(5)
Pages that refer to this page: nscd(8)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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